The present invention relates to a moisture measuring apparatus and technique and, more specifically, to the use of microwave energy to measure the percent moisture in a predetermined volume.
The use of microwave energy to aid in the determination of moisture content in different materials is well known in the art. In such devices and processes, microwaves are propagated through a particular material and are attenuated by an amount dependent on the quantity of moisture in the product. At the same time, gamma or other radiation is directed through the material and also attenuated depending upon the density of the material. With an appropriate combination of the two measurements of microwave and other radiation, a reading approximating the percent moisture of the sample can be obtained. Naturally, the accuracy of any reading will be dependent upon the accuracy of detecting the attenuation of the microwave and other radiation signals transmitted through the sample material.
Apparatus employing the above techniques are disclosed, for example, in U.S. Pat. Nos. 3,460,030 and 4,131,845. In the exampled instances, the measurements are made in connection with material moving in a chute or trough to enable a continuous measurement of the percent moisture content in processed material. Although such devices have enabled the calculation of an approximate percentage of moisture content, they have been somewhat complex and not suitable for making inexpensive measurements of discrete quantities of sampled material.
In U.S. Pat. No. 3,499,499, rather than determine the percent moisture content of a material, microwaves are used to determine the water weight so that adjustments in the dry weight of a material can be made. This technique enables the compensation for moisture in material, but does not provide for a direct reading of the percent moisture contained in any given sample.
The device in U.S. Pat. No. 3,499,499 also lacks two features which would be needed for accurate moisture analysis with microwave methods. It is well known in the art that microwave characteristics of water are affected by temperature. No means of measuring or compensating for temperature effects are provided. In addition, repeatability and accuracy of moisture measurement is greatly enhanced if microwave propagation in the measurement zone is optimized. U.S. Pat. No. 4,131,845 is particularly concerned with providing microwave absorbent material on the inside of the chute sensor to adjust microwave characteristics of the sensor for best measurement accuracy.
In still other instances, the percent moisture content in a given sample has been calculated by an evaporative technique. In those instances, the sample is first weighed and then subjected to heat to cause the evaporation of moisture in the sample. The sample is then weighed again and the difference in material weight preceding and following the drying enables the calculation of the percent moisture content. Such processes, however, normally require a long drying time to prevent the possibility of burning the material. In addition, in order to decrease such drying time, the samples are often ground or otherwise altered and the heat generated during such alteration varies the original moisture content resulting in an inaccurate final reading. Finally, during the drying process, other volatiles in the material may be evaporated, thereby resulting in an inaccurate final moisture reading.
In an attempt to increase the speed of the above drying technique, some techniques have employed microwave radiation to perform the drying process. While this technique improves the drying time, and works satisfactorily with relatively high moisture materials, the effectiveness of the microwave drying significantly decreases as the moisture content of the material decreases. This condition is caused by the decrease in water molecules with which the microwave energy may interact at the lower moisture contents. Restrictions on the use of particular microwave frequencies for drying also prevent the use of optimum frequency with the water molecule. This, likewise, reduces the sensitivity to materials with low moisture content.
The present invention has been developed to overcome the specific shortcomings of the above known and similar techniques and to provide for microwave measurement of percent moisture in discrete quantities of material.